Welding fluxes are generally in one of three forms. They may be either fused, agglomerated or bonded.
The fused is formed by melting the flux at temperatures between 2700 and 3100 F before chilling and fragmenting the flux. The agglomerated flux has a ceramic binder which is cured at temperatures of about 2550 F. In producing a bonded flux, the raw materials are ground, dry mixed and then bonded together with the addition of potassium silicate or sodium silicate. The mixture is then pelletized and cured at relatively low temperature to drive off the moisture. Due to the relatively low temperature of the curing, metallic deoxidizers and ferroalloys can be included in the flux, without being destroyed by high temperature curing. One disadvantage of the bonded flux is that it is more likely to absorb moisture than the other types and a second disadvantage is that removal of fines, either willfully or inadvertently, will effect some alteration of the flux composition which does not occur with the other type fluxes.
The conventional method of curing the pelletized bonded flux comprises feeding the material into a high heat rotary kiln. This process takes approximately 40 minutes from the time the flux enters the kiln until exit. Due to the tumbling action within the kiln and the velocity of gases passing through the kiln, many of the fines are inadvertently removed during this process. This is sometimes minimized by making large pellets, which then requires subsequent grinding to the desired size.
Rotary kilns tend to be relatively massive and less than conveniently portable. They are costly in maintenance and fuel consumption and a typical kiln has to be fired for a period of 18 to 24 hours so that the temperature can be brought up gradually. Since these disadvantages occur even in small kilns it has been the tendency to restrict operations to large size kilns which handle a large volume of flux in a single batch.